Krios Battle Tank
The Krios Battle Tank is the main battle tank of the Adeptus Mechanicus, and unlike most Imperial Tank designs it is horrendously exposed, what with the cogs and gears being placed everywhere for the eye to see. You would think the Tech Priests would have the dignity to armor up their main battle tanks so that it could survive more than a few minutes in a war. Seriously who the hell designed this! (Well, it's not like we haven't seen laughably insufficient tank suspension or ridiculously exposed treads on a war machine of the imperium before...)
Ranting aside, the Krios is an ancient and arcane tank that is durable and protected by interlocking energy fields (Which I believe off-sets its horribly exposed mechanics, although that is still not an excuse). Its most common armament is the Lightning Cannon which can fuck over most enemy vehicles and tanks, making it akin to a Space Marine Predator, except more incompetently designed.
On the other hand, its closer to a SPG (self-propelled gun) than a tank in design: its just a gun on treads, and its meant to go fast and flank rather than slug it out. Its also mentioned that its got a pretty big and powerful reactor for its size, and that it uses this to both keep its super-charged engines running and its cannon firing. Presumably its open design is for cooling, since the combination of hot engine and hot gun will melt it into scrap, causing the tending tech priest to have an aneurysm (or BSOD).
The main weapon for the Krios is the Lightning Cannon, a laser-guided electromagnetic beam designed to focus on a target and fucking melt it. It has a pretty short range, though, so the lack of a turret means that it takes some work to compensate for that. But, you don't need to worry about ammo and compared to a las-cannon, it has a much higher rate of fire.
Yes, believe it or not, the Krios has another brother called the Krios Venator which changed the role of the Krios from a MBT to a dedicated tank destroyer, swapping its Lightning Cannon with a Pulsar-Fusil (Think a multi-chambered particle beam cannon designed to fire volleys of destructive energy bolts which detonate explosively upon hitting their target), like most Super OP weaponry, the Pulsar-Fusil is guarded by the Mechanicus, only using when they are dragged into a conflict.